What is a sinkhole

They go by called many names (snake hole, swallow hole, or doline), they’re big, they appear seemingly out of nowhere, and they can “devour” houses in the blink of an eye. But in reality, they develop over many years and require very specific conditions and processes to form.

They are natural depressions (or holes) on the Earth’s surface, (usually) caused by karst processes. Karst processes occur when bedrocks are soluble – in other words, in 99% of all cases, in carbonate rocks (like limestone or dolomite) or evaporitic rocks (like gypsum or anhydrite). So while they can occur in other environments, I wouldn’t really worry about it.

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A possible mechanism for sinkhole formation

A sinkhole has no natural external surface drainage – when it rains, all the water stays inside the sinkhole and typically drains into the subsurface. This is the dominant phenomena regardless of size – it can be smaller than a meter, or over 100 meters in depth (In Sarisarinama, Venezuela, multiple sinkholes have reached about 1,000 feet wide and 1,150 feet deep). They can also have very different shapes; some are just like a shallow saucer or bowl, while others are much more vertical. Some actually form water, and you’ve probably seen this in the form of little ponds in limestone.

What really makes them dangerous is the fact that they’re really unpredictable. They form so slowly, that without thorough geological or geophysical research, you can’t really tell if something’s changing – this is why collapses have dramatic, unexpected effects, especially in urban settings.

Areas prone to sinkhole collapses

As I said, sinkholes are almost always prone to karst areas. In such areas, it’s possible to see hundreds or even thousands of sinkholes in a small area, and no surface streams, because all the drainage occurs subsurface. Evaporite rocks underlie about 35 to 40 percent of the United States, but in most cases, they are buried very deep, so there are only a few threatened areas.

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The most impressive sinkholes form in thick layers of homogenous limestone. Their formation is facilitated by high groundwater flow, often caused by high rainfall. When these thick, relatively soluble layers sit on top of insoluble rock, significant underground streams (or even rivers) may form, dissolving a significant quantity of rock, creating large underground voids.

Sinkholes can also be underwater. They’re typically called blue holes and have been described especially in the Bahamas area. The name originates from the deep blue color of the water in these sinkholes, which in turn is created by the high lucidity of water and the great depth of sinkholes; only the deep blue color of the visible spectrum can penetrate such depth and return back after reflection. The deepest known sinkhole is the Dean’s Blue Hole in the Bahamas.

Types of sinkholes

artificial sinkholes;

dissolution sinkholes;

cover-subsidence sinkholes;

cover-collapse sinkholes.

Sinkholes can be created by humans!

I wanted to get this clear right off the bat. This type of sinkhole is actually quite common (not as common as natural ones, but still relevant, especially in urban areas). They can be caused by a number of human activities, most notably groundwater pumping and construction and development practices. The most common activities which lead to this type of phenomena are drilling, mining, significant changes in weight (lots of construction in a relatively pristine area), and a heavy increase in water flow (like say, a formation of an artificial pond, pipe leakage, etc.

Basically, either more water appears in the system, which dissolves and creates an underground void, or the underground is already present, and there is an increase on the pressure exerted on the surface – or a combination of these. Whenever the structural and chemical balance is disturbed – sinkholes can occur.

Dissolution sinkholes

As the name describes, dissolution (the process of dissolving a solid substance into a solvent to make a solution) is the driving factor here. Dissolution of the limestone or dolomite is most intensive where the water first contacts the rock surface.

Cover-subsidence sinkholes

Cover-subsidence sinkholes tend to develop gradually where the covering sediments are permeable and contain sand. Not really your typical sinkhole, this type is not common, and can often go undetected for long periods of time – which is pretty much the only thing that can make them dangerous.

Cover-collapse sinkholes

These are pretty much the most dangerous type of sinkhole. Cover-collapse sinkholes develop very fast (sometimes even in a matter of hours), and can have catastrophic damage. They occur where the covering sediments contain a significant amount of clay; over time, surface drainage, erosion, and deposition of sinkhole into a shallower bowl-shaped depression. To put it in simple terms, the ground simply can’t support the load ontop of it.

So how common are sinkholes, really?

The thing is, unlike earthquakes or hurricanes, sinkholes aren’t really tracked. Also, since they appear in so many areas, it’s practically impossible to discuss where they appear – we can only talk about areas prone to them. As I said, any karst-dominated area has some, be they big or small, as do areas with surface evaporites. This is why big portions of the Middle East, for example, are just riddled with sinkholes. In the US, Florida is almost certainly the most threatened state. From what I can dig up, from 2006 to 2010 in Florida alone, there were 24,671 insurance claims for sinkholes.

So there’s no need to worry if you don’t live in one of these areas. If you do, however, look for certain tells, such as sagging trees or fence posts, doors or windows that won’t close properly, and rainwater collecting in unusual spots. If you’re worried, then you should consult a professional geologist or geotechnical engineering firm that can explain the situation. If indeed there is a possibility of a sinkhole near your home, they can even inject a specific concoction to fill up the cracks and strengthen the foundation.

Andrei's background is in geophysics, and he's been fascinated by it ever since he was a child. Feeling that there is a gap between scientists and the general audience, he started ZME Science -- and the results are what you see today.